The idea that light interacts with the "vacuum" and so its speed changes is getting more and more popular:

http://blogs.voanews.com/science-world/2013/03/26/speed-of-light-may-not-be-constant/"Speed of Light May Not be Constant (...) Two separate studies by scientists from the University of Paris-Sud in France and from the Max Planck Institutes for the Physics of Light in Germany are disputing the long established belief concerning the nature of a vacuum. (...) A vacuum, when viewed at the quantum level ? at the smallest and most basic level ? is not empty, but instead filled with particle pairs such as electron-positron or quark-antiquark pairs that are constantly appearing and disappearing. While these particle pairs are real particles, their lifetimes are extremely short. If these findings are proved to be true, they could have an impact on current scientific theories that take the speed of light into consideration. Both studies will be published in an upcoming edition of the European Physical Journal - D (EPJ-D)."